Arrangement and method for forming a multilayered paper or paperboard web

ABSTRACT

An arrangement and method for forming a multilayered paper or paperboard web from stock suspensions containing papermaking fibers and fines, in which first and second webs are formed in first and second twin-wire forming units such that each web has a higher concentration of fines at one side than at the other side of the web. The webs are couched together such that the sides having the higher concentration of fines are in contact with each other.

TECHNICAL FIELD

[0001] The present invention relates to an arrangement for fanning amultilayered paper or paperboard from stock suspensions with papermakingfibres and fines, said arrangement including at least a first and asecond forming unit, at least one of the forming units being of twinwire type and having a forming roll for forming a fibrous web having ahigher content of fines at one side than at the other, and a couchdevice for couching together said fibrous web having a higher content offines at one side with an other fibrous web to form the multilayeredweb.

[0002] The invention relates to a method of a multilayered paper orpaperboard web from stock suspensions with papermaking fibres and fines,said method including the steps of:

[0003] forming a first fiber web having a higher content of fines at oneside than at the other in a first forming unit of twin wire type andhaving a forming roll,

[0004] forming a second fiber web in a second forming unit, and

[0005] couching the formed webs together to form the multilayered web.

[0006] The invention is particularly advantageously implemented whenforming testliner, fluting, liner, white-top liner, gypsum board liner,and paperboard based on recycled pulp fibres. However, the inventionalso can be implemented when forming other paper or paperboard webs.

BACKGROUND OF THE INVENTION

[0007] When manufacturing multilayer liner and paperboard, it ispreviously known to form bottom layer and top layer(s) for example inso-called fourdinier forming units, or alternatively in twin-wireforming units of a paper or paperboard machine, in order to subsequentlycouch the layers together into a coherent web, which is pressed in apress section and dried in a drying section.

[0008] It is also previously known that forming the top and bottomlayers in fourdinier forming units, as a rule, results in a finishedproduct with a higher internal bond strength, i.e. bond strength betweenthe layers, than what is achievable if the top and bottom layers areformed in twin-wire forming units.

[0009] The patent publication DE 2 020 688 discloses a method forjoining two webs of paper, paperboard or the like, more specifically acouching method, and a device for implementing the method. The methodcomprises that the two webs ran in the same direction during theirmanufacture and are joined in a such a way that the wire-side of thesecondary web comes to rest on the top side of the primary web.

[0010] The device disclosed in DE 2 020 688 comprises a movable longwire (fourdinier) as a primary wire, and at least one wire arranged incirculation above the, long wire as a secondary wire. In the lowestportion of its path, each wire runs in the opposite direction inrelation to the long wire, wherein each wire in circulation has beensupplied with a transfer roll which is arranged so that it contacts thewire in circulation on an outer side and the long wire on its top side.Thereby, according to DE 2 020 688, the advantage is obtained thatalways one surface of a paper web manufactured through the method willexhibit the properties of a sheet top-side, such as an improvedprintability in comparison with a wire-side wherein this is also thecase if more than two paper webs are joined by means of the method.

[0011] The patent publication DE 40 31 038 A 1 discloses a device formanufacturing a multilayer paper or multilayer paperboard by means of aprimary wire, and a secondary wire arranged above this as twin-wireforming unit, which forms a top layer of the multilayer paper orpaperboard onto a bottom layer. Thereby, the twin-wire forming unit hasa forming roll as a breast roll for configurating the twin-wires.Upstream the forming roll, a pressure application unit is arranged,wherein the waterline of the pulp suspension is within the region of theforming roll. According to DE 40 31 038 A 1, amongst other things, thedisclosed device makes it possible to obtain a higher internal bondstrength, since the dry content of the top layer is relatively low whenthe couching to the bottom layer takes place.

[0012] Furthermore, the patent publication U.S. Pat. No. 5,607,555discloses a forming section of a paper machine having two twin-wireforming units for forming a multilayer web. Each twin-wire forming unitconsists of a headbox, two wire loops arranged to define a common wirepath for molding the web being formed in a sandwich-like manner, anddrainage elements for each of the wire loops. The common wire path ofeach forming unit has a first section including a curved suctiondrainage element or roll in the first lower wire loop, and a secondsection including opposing drainage ledges. Thereby, the drainage ledgesare stationary at the upper side and designed to be resilientlypressable on the bottom side. The drainage ledges of the upper and lowersides are arranged staggered with respect to each other in the directionof travel of the wire wherein at least the ledges of the upper side aredesigned for suction. Furthermore, there is at least one third sectionwhich has at least one suction wire-separating element on one side. Thetwin-wire forming units are arranged so that the same drainage elementsare associated in each case with what will be the inner and the outersides of each layer in the multilayer web. According to U.S. Pat. No.5,607,555, a very uniform paper is obtained in this way. Furthermore,its is reported that the contents of filler and/or fines can bedisplaced towards the outsides of the layers of the paper web by meansof arranging the drainage ledges in a suitable way, depending on theprevailing requirements. As a result of this, a larger number of fibresare believed to be present on the two sides of the layers which restagainst each other, wherein a better adhesion between the layers isobtained when they have been couched together.

[0013] However, it has been found that multilayered paper or paperboardwhich has been formed in the previously known arrangements can have aninsufficient internal bond strength in the z-direction.

[0014] One reason for this is that many of the previously knownarrangements utilise drainage elements of such types which generate veryhigh shear forces in the pulp suspensions which are dewatered. Examplesof such drainage elements are vacuum forming rolls and suction boxeswith drainage toils generating a high vacuum. High shear forces in thepulp suspensions result in a low retention of the fine material (fines)which is important in order to obtain a high internal bond strength, orat least result in the fines content being strongly reduced at the sideof a paper or paperboard layer which has been facing such a drainageelement during the dewatering. This often makes it difficult to obtain asufficient internal bond strength of the finished paper or the finishedpaperboard.

SUMMARY OF THE INVENTION

[0015] Accordingly, a first object of the present invention is toprovide an arrangement for forming a multilayered paper or paperboardweb, which arrangement maximises the amount of fines which is retainedwhen forming the layers included in the web, and ensures that thosesides of the layers which have the highest content of fines are couchedtogether with each other, so that the multilayered paper or paperboardweb obtains a raised internal bond strength.

[0016] In accordance with claim 1, this first object of the presentinvention is achieved in that both of the forming units are of twin wiretype and include a forming roll for forming a fibrous web having ahigher content of fines at one side than at the other, and that the twoforming are so arranged relative to each other as to place the web sideshaving the highest content of fines in contact with each other forcouching together to form the multilayered web.

[0017] According to a preferred embodiment of the invention, the formingrolls are open and adapted for making the content of fines in the fibrewebs higher at the inner-wire side than at the outer wire-side duringthe initial dewatering, wherein the additional drainage members areadapted for maintaining the content of fines at least substantiallyunchanged at a high level at the inner-wire side during the continueddewatering, and the forming units art designed and arranged in relationto each other so that the two formed fibre webs are joined with theinner-wire sides against each other.

[0018] A second object of the present invention is to provide a methodfor forming a multilayered paper or paperboard web which methodmaximises the amount of fines that is retained when forming the layersincluded in the web, and ensures that those sides of the layers thathave the highest content of fines arc couched together with each other,so that the multilayer paper or paperboard web obtains a raised internalbond strength.

[0019] In accordance with claim 14, this second object of the presentinvention is achieved by:

[0020] a using as second forming unit a twin wire forming unit having aforming roll,

[0021] forming also the second fiber web with a higher content of finesat one side than at the other, and

[0022] positioning the webs so as to ensure that those sides of thewebs, which have the highest content of fines, are couched together toform the multilayered web.

[0023] According to a preferred embodiment of the invention, the methodadditionally includes the steps of choosing open forming rolls for theinitial dewatering, adapting them so that the content of fines in thefibrous webs becomes at least as high at the inner wire side as at theouter wire side during the initial dewatering, adapting the additionaldrainage members so that the content of fines will be maintained atleast substantially unchanged at a high level at the inner wire sideduring the continue dewatering, and joining the two formed fibrous webswith the inner wire sides against each other.

[0024] It should be noted that the expression “open forming rolls”, asused herein, means that a two-sided dewatering on the forming rolls, ifpossible, is accomplished without the application of any externalsuction.

[0025] When the wire speed of a twin-wire forming unit is increased, asis well-known to the skilled person, a stage will be reached where theg-force becomes so large that the inwardly directed pressure generatedby the tension in the outer-wire no longer is able to press any waterthrough the inner-wire, so that all dewatering will take place throughthe outer-wire. In such cases, it may become necessary to generate anegative pressure within the forming roll in order to be able to directat least part of the dewatering through the inner-wire. An entirelysymmetrical dewatering would be possible to achieve only in cases wherethere were no g-forces at all.

[0026] For this reason, the expression “open forming rolls”, as usedherein, is also intended to include embodiments of the invention where asuction forming roll with a low, or at the most, a moderate negativepressure is utilised in order to prevent all dewatering from takingplace through the outer-wire at high wire speeds, but in whichembodiments the main drainage impulse still is generated by the wiretension during the passage of the wires over the carved peripheralsurface of the forming roll.

[0027] Further objects of the present invention will become evident fromthe following description, while the features which enables the furtherobjects to be achieved are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the following, the invention will be described in greaterdetail with reference to the attached drawings, in which

[0029]FIG. 1 shows a simplified and schematic side view of anarrangement according to the invention in a first advantageousconfiguration,

[0030]FIG. 2 shows a simplified and schematic side view of anarrangement according to the invention in a second advantageousconfiguration,

[0031]FIG. 3 shows a simplified and schematic side view of anarrangement according to the invention in a third advantageousconfiguration,

[0032]FIG. 4 shows a simplified and schematic side view of anarrangement according to the invention in a fourth advantageousconfiguration,

[0033]FIG. 5 schematically illustrates some main steps in a methodaccording to a preferred embodiment of the invention.

[0034]FIG. 6 in diagram form illustrates the relative percentage offines as a function of the thickness Z of a first fibre web from bottomBO to top TO, wherein the first fibre web has been formed in a formingunit of an arrangement according to the invention, and a test specimenfor analysis of the fines content has been extracted before the joiningto a second fibre web, and

[0035]FIG. 7 shows a simplified and schematic side view of anarrangement according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] In the following, a number of different embodiments of thearrangement according to the invention will be described, and whenapplicable with reference to the attached FIGS. 1-7.

[0037] The arrangement according to the invention comprises a first 101,201, 301; 401 and a second forming unit 102, 202; 302; 402, wherein eachforming unit is of a twin-wire type and has a forming roll 109; 209;309; 409 and 111, 211; 311; 411, and an inner-wire 105: 205; 305; 405and 107; 207; 307; 407 and an outer-wire 106; 206; 306; 406 and 108;208; 308; 408 enclosing a portion of the circumference of eachrespective forming roll and thereby forming an initial, curved sectionof a forming zone for an initial dewatering and forming of a fibre web Aand B, having an inner-wire side x and z supported by the inner-wire andan outer-wire side y and w supported by the outer-wire The curvedsection is followed by a section with additional drainage members 110;210; 310; 410 and 112; 212; 312; 412 for a continued dewatering of theformed fibre web A and B, wherein vacuum transfer means 113, 114, 115;213, 214, 215; 313, 314, 315, 327; 413, 414, 415, 427 are arranged forjoining the fibre webs A, B in order to form a multilayered fibre web C,which is intended to be included in or to constitute the multilayeredpaper or paperboard web D after pressing and drying.

[0038] Thereby, according to the invention, the forming rolls 109, 111;209, 211; 309, 311; 409, 411 are open, and adapted to make the contentof fines in the fibre webs A, B higher at the inner-wire side x and zthan at the outer-wire side y and w during the initial dewatering, whilethe additional drainage members 110, 112; 210, 212; 310, 312, 410, 412are adapted for retaining the content of fines at least substantiallyunchanged at a high level at the inner-wire side x and z during thecontinued dewatering, and the forming units 101; 201; 301; 401 and 102;202; 302; 402 are designed and arranged in relation to each other sothat the two formed fibre webs A, B are joined with the inner-wire sidesx and z against each other.

[0039] The above-mentioned open forming rolls of the arrangementaccording to the invention are adapted in order to provide a “gentle”dewatering, substantially without any shear forces, contrary to what isthe case with vacuum or suction forming rolls of arrangements accordingto prior art, and enabling a high proportion of the supplied fines inthe pulp suspensions to remain in the suspensions all the way to thecontinued dewatering. However, a certain redistribution of the fines ina direction towards the forming rolls can occur in the pulp suspensions.

[0040] The specially adapted additional drainage members of thearrangement according to the invention do not comprise any elements atthe inner-wire sides which generate high shear forces, such as drainagefoils and the like. This is the reason why the main proportion of thefines are retained at the inner-wire sides also during the continueddewatering. During the continued dewatering, a minor portion of thefines can be suctioned out from the pulp suspensions on the side, i.e.the outer-wire side, where the additional drainage members preferablyall are arranged in the arrangement according to the invention. Thearrangement according to the invention maximises the quantity of fineswhich is retained when forming the layers included in the web, andensures that those sides of the layers which have the highest content offines, i.e., the inner-wire sides, are couched together with each otherso that the resulting multilayered paper or paperboard web obtains anincreased internal bond strength in comparison to paper or paperboardwebs which have been formed in arrangements according to prior art.

[0041] In a particularly preferred embodiment, the above-mentionedadditional drainage members 110, 112, 210, 212; 310, 312; 410, 412 arearranged for completing the forming of the fibre webs A and B. In thiscontext, the forming can be regarded as completed when the main part ofall free water has been removed, so that a coherent paper sheet has beenformed. In this embodiment, the additional drainage members are allarranged at the outer-wires 106, 108; 206, 208; 306, 308; 406, 408 fordewatering the fibre webs from the outer wire-sides y and w.

[0042] The above-mentioned additional drainage members 110, 112; 210,212; 310, 312; 410, 412 particularly advantageously comprise vacuumsuction boxes with one or several vacuum zones adapted in order toremove substantially all free water from the fibre webs A and B beforethe joining. However, within the scope of the invention, it is alsoconceivable with embodiments with other types of additional drainagemembers.

[0043] In one advantageous embodiment of the arrangement 500 accordingto the invention, particularly illustrated by FIG. 7, the forming rolls509, 511 have a diameter of about 1.4-1.8 m, and are arranged with awire winding angle which is at least 80° for the initial dewatering.Thereby, one or several of the vacuum transfer means 513, 514 arearranged in order to constitute the additional drainage members for thecontinued dewatering. Accordingly, the arrangement according to thisembodiment comprises forming rolls having a large diameter and a largewire winding angle, something which provides an efficient dewatering anda very high dryness already after the initial dewatering. Thisembodiment enables a simplified arrangement, since the continueddewatering and the web transfer between the wires are provided by thesame members.

[0044] According to another advantageous embodiment, the arrangementcomprises headboxes 103, 104; 203, 204; 303, 304; 403, 404, formingrolls 109, 111; 209, 211; 309, 311; 409, 411, additional drainagemembers 110, 112; 210, 212; 310; 312; 410, 412, and preferably alsowires 105, 107; 205, 207; 305, 306, 307, 308; 405, 406, 407, 408, whichare identical in the first 101; 201; 301; 401 and in the second 102;202, 302; 402 forming unit. This embodiment provides large costadvantages, since the number of spare parts in stock can be minimised.

[0045] In an even more advantageous embodiment, the arrangement furthercomprises drive rolls 116, 117; 216, 217; 316, 317; 416, 417, guiderolls 118, 119; 218, 219; 318, 319; 418, 419 and tensioning rolls 120,121; 220, 221; 320, 321; 420, 421 which are identical in the first 101;201; 301; 401 and the second 102; 202; 302; 402 forming unit. Thereby,it is of course also conceivable with embodiments in which the guiderolls are driven, or where the tensioning rolls function as guide rolls.This embodiment enables very low costs for keeping spare parts in stock.

[0046] In a particularly advantageous embodiment, the first 101; 201;301; 401 and the second 102; 202; 302, 402 forming unit aresubstantially mirror-invertedly identical.

[0047] In a first alternative embodiment, particularly illustrated byFIGS. 1 and 2 together with FIG. 5, the arrangement further comprises afirst vacuum transfer means 113; 213 arranged for transferring a firstfibre web A from the inner-wire 105; 205 to the outer-wire 106; 206 ofthe first forming unit 101; 201, a second vacuum transfer means 114; 214arranged for transferring a second fibre web B from the inner-wire 107;207 to the outer-wire 108; 208 of the second forming unit 102; 202, anda third vacuum transfer means 115; 215 arranged for transferring thesecond fibre web B from the outer-wire 108; 208 of the second formingunit 102; 202 onto the first fibre web A in order to form themultilayered fibre web C. In the described embodiment, these vacuumtransfer means does not have as their task to achieve any dewatering,they are only intended to transfer a fibre web from one wire to another.

[0048] In a second alternative embodiment, particularly illustrated byFIGS. 3 and 4 together with FIG. 5, the arrangement 300; 400 comprises abottom layer forming unit 328; 428 having an additional headbox 329; 429for forming a bottom layer E onto at least one bottom-wire 330; 430,wherein at least one vacuum transfer means 313; 413 is arranged fortransferring the first fibre web A onto the bottom layer E on thebottom-wire 330; 430. In this embodiment, the arrangement 300; 400particularly advantageously comprises at least one additional vacuumtransfer means 327; 427 arranged for transferring the first fibre web Afrom the outer-wire 306; 406 to the inner-wire 305; 405 of the firstforming unit 301; 401. The bottom layer forming unit of course alsocomprises suitable drainage members 331; 431.

[0049] In a third, alternative embodiment, particularly illustrated byFIGS. 2 and 4 together with FIG. 5, the arrangement 200; 400 furthercomprises a top layer forming unit 222; 422 having an additional headbox223; 423 for forming a top layer F on at least one top-wire 224; 424,and an additional vacuum transfer means 226; 426 arranged fortransferring the top layer F onto the second fibre web B in order toform the multilayered fibre web C. Also the top layer forming unit ofcourse comprises suitable drainage members 325; 425.

[0050] The above-mentioned vacuum transfer means or additional vacuumtransfer means of the arrangement according to the invention preferablycomprises vacuum suction boxes 113, 114, 115; 213, 214, 215, 226; 313,314, 315, 327; 413, 414, 415, 426, 427 and/or vacuum suction rolls.

[0051] In the following, a number of different embodiments of a methodfor forming a multilayered paper or paperboard web according to theinvention will be described, when applicable with reference to theattached FIGS. 1-7.

[0052] The method according to the invention comprises that pulpsuspensions with pulp fibres and fines are dewatered in a first 101;201; 301; 401 and a second 102; 202; 302; 402 forming unit. Each formingunit is of a twin-wire type and has a forming roll 109; 209; 309; 409and 111; 211; 311; 411, and an inner-wire 105; 205; 305; 405 and 107;207; 307; 407 and an outer-wire 106; 206; 306; 406 and 108; 208; 308;408 enclosing a portion of the circumference of each forming roll, andthereby forming an initial, curved section of a forming zone in whichinitial dewatering and forming is performed of a fibre web A and Bhaving an inner-wire side x and z supported by the inner-wire and anouter-wire side y and z supported by the outer-wire. Thereafter, in asection downstream the curved section, a continued dewatering of theformed fibre web A, B is performed by means of additional drainagemembers 110; 210; 310; 410 and 112; 212; 312; 412, whereafter the fibrewebs A, B are joined by means of vacuum transfer means 113, 114, 115;213, 214, 215; 313, 314, 315, 327; 413, 414, 415, 427 in order to form amultilayered fibre web C, intended to be included in or to constitutethe multilayered paper or paperboard web D after pressing and drying.

[0053] According to the invention, open forming rolls 109, 111; 209,211; 309, 311; 409, 411 are chosen for the initial dewatering and areadapted so that the content of fines in the fibre webs A, B becomes atleast as high at the inner-wire side x and z as at the outer-wire side yand w during the initial dewatering, while the additional drainagemembers 110, 112; 210, 212; 310, 312; 410, 412 are adapted so that thecontent of fines is maintained at least substantially unchanged at ahigh level at the inner-wire side x and z, and wherein the two formedfibre webs A, B are joined with their inner-wire sides x and z againsteach other.

[0054] In an advantageous embodiment of the method according to theinvention, the initial dewatering results in a dry content of the pulpsuspensions which is between 2 and 3%.

[0055] In a particularly preferred embodiment of the method according tothe invention, the continued dewatering primarily takes place in adirection towards the outer-wire sides y and w of the fibre webs A andB. In this embodiment, the highest possible content of retained fines isobtained at the inner-wire sides x and z, which thereafter are to becouched together, and consequently also the highest possible internalbond strength.

[0056] The continued dewatering particularly advantageously results in adryness of the fibre webs A and B which is higher than about 6 to 8%. Atsuch a relatively high dry content, the forming can be regarded ascompleted, and the fines present in the fibre webs are retained in astable way even in case the fibre webs subsequently would be subjectedto high vacuum (i.e. suction).

[0057]14

[0058] In one advantageous embodiment of the method according to theinvention, particularly illustrated by FIG. 7, forming rolls 509, 511having a diameter of about 1.4-1.8 m are chosen. Thereby, the wirewinding angle is adapted to be at least 80%, wherein the initialdewatering results in a dry content of the pulp suspensions in the orderof 6%. The continued dewatering results in a dry content of the fibrewebs A and B which is higher than about 6-8%, wherein the continueddewatering is accomplished by means of one or several of the vacuumtransfer means 513, 514.

[0059] In another advantageous embodiment of the method according to theinvention, a few of the vacuum transfer means 113, 114; 213, 214; 313,314, 327; 413, 414, 427 provide an additional dewatering after thecontinued dewatering, resulting in a dryness higher than about 8 to 10%of the fibre webs A and B before they are joined into the multilayeredfibre web C. In this embodiment, the high content of fines at theinner-wire sides x and z which is obtained by means of the methodaccording to the invention makes it possible to obtain a high internalbond strength.

[0060] In a first alternative embodiment of the method according to theinvention, particularly illustrated by FIGS. 1 and 2 together with FIG.5, a first fibre web A is transferred from the inner-wire 105; 205 tothe outer-wire 106; 206 of the first forming unit 101; 201, while asecond fibre web B is transferred from the inner-wire 107; 207 to theouter-wire 108; 208 of the second forming unit 102; 202 in order to betransferred therefrom be onto the first fibre web A.

[0061] In a second alternative embodiment of the method according to theinvention, particularly illustrated by FIGS. 3 and 4 together with FIG.5, the method further comprises to form a bottom layer E on at least onebottom-wire 330; 430, and that the first fibre web A thereafter istransferred onto the bottom layer E. In this embodiment, the first fibreweb A is particularly advantageously transferred from the outer-wire306; 406 to the inner-wire 305; 405 of the first forming unit 301; 401before the first fibre web A is transferred.

[0062] In a third alternative embodiment of the method according to theinvention, particularly illustrated by FIGS. 2 and 4 together with 5,the method further comprises to form a top layer F on at least onetop-wire 224; 424, and that the top layer thereafter is transferred ontothe second fibre web B.

[0063] The present invention should by no means be regarded as beinglimited to what has been disclosed above in connection with thedifferent embodiments, or to what is shown in the attached drawings, butthe scope of the invention is defined by the following claims.

[0064] In FIG. 1, an arrangement 100 according to the invention is shownin greater detail in a configuration which is particularly well suitedfor forming testliner or fluting having a top layer within the grammagerange 30-100 g/m² and a base layer within the range 50-150 g/m².However, the arrangement could also be used when forming other types ofpaper or paperboard webs in cases where it is desired to form and couchtogether two different layers. As is evident from FIG. 1, a very compactand cost efficient unit is obtained in this configuration, since anumber of the parts included in the two twin-wire forming units 101, 102are identical in both units. Since the forming rolls 110, 111 are openand are not vacuum suction rolls, no significant redistribution of finestakes place in the pulp suspension, and a relatively uniform sheetstructure in the Z-direction can be obtained. Furthermore, during theforming, the additional drainage members 110, 112 are acting exclusivelyon the outer-wire sides y and w, B, which are not to be couchedtogether, of the fibre webs A and, for this reason, the content of finesat the opposing inner-wire sides x and z, which are to be couchedtogether, remains unchanged at a high level (see FIG. 5).

[0065]FIG. 2 shows an arrangement 200 according to the invention ingreater detail in a configuration which is particularly well suited forliner, “white top” liner, or gypsum board liner. However, thisarrangement could also be utilised when forming other types of paper orpaperboard webs where it is desirable to form and couch together threedifferent plies. The arrangement 200 can be said to be of the type“Top-side Down”, since a top layer is formed in the twin-wire formingunit 201, a middle layer is formed in the twin-wire forming unit 202,and a bottom layer is formed in the fourdrinier forming unit 222. Thegrammage of the top layer advantageously can be 40-80 g/m², of themiddle layer 55-170 g/m², and of the bottom layer 30-50 g/m², but it isalso conceivable with other grammage intervals.

[0066]FIG. 3 shows another arrangement 300 according to the invention ingreater detail, also in a configuration which is particularly wellsuited for liner, “white top” liner or gypsum board liner. However, thisarrangement could also be utilised when forming other types of paper orpaperboard webs where it is desired to form and coach together threedifferent layers. In contrast to the arrangement in FIG. 2, thearrangement in FIG. 3 is of the type “Top-side Up”, i.e. a bottom layeris formed in the fourdinier forming unit 328, a middle layer is formedin the twin-wire forming unit 301, and a top layer is formed in thetwin-wire forming unit 302. The grammage of the bottom layeradvantageously can be 43-80 g/m², of the middle layer 55-155 g/m², andof the top layer 40-65 g/m², but it is also conceivable with othergrammage intervals.

[0067]FIG. 4 discloses an arrangement 400 according to the invention ingreater detail, now in a configuration which is particularly well suitedfor liner or paperboard, and particularly for paperboard based onrecycled fibres. However, this arrangement could also be utilised whenforming other types of paper or paper webs where it is desirable to beable to form and couch together four different layers (sheets). In thearrangement in FIG. 4, a bottom layer is formed in the fourdinierforming unit 428, a single middle layer or alternatively a first middlelayer is formed in the twin-wire forming unit 402, an underliner oralternatively a second middle layer is formed in the twin-wire formingunit 402, and a top layer is formed in the fourdrinier forming unit 422.The grammage of the above-mentioned bottom layer can advantageously be25-90 g/m², of the above-mentioned single middle layer 40-200 g/m², ofthe above-mentioned first middle layer 60-180 g/m², of theabove-mentioned underliner 30-90 g/m², of the above mentioned secondmiddle layer 60-180 g/m², and of the above-mentioned top layer 30-60g/m². However, it is also conceivable with other grammage intervals.

[0068] When the different components included in the arrangementaccording to the invention are concerned, these can but do notnecessarily have to be of a construction and function previously knownper se, for example applicable components which are disclosed in theabove-discussed documents with prior art.

[0069] In the form of a diagrams the attached FIG. 6 illustrates therelative percentage of fines F as a function of the thickness Z of afirst fibre web from the bottom BO to the top TO, wherein the firstfibre web has been formed in a forming unit of an arrangement accordingto the invention, and a specimen for analysis of fines content has beenextracted before joining to a second fibre web.

[0070] As is evident from FIG. 6, the relative proportion of fines atthe bottom BO of the specimen is only slightly lower than the maximumproportion 100% slightly further into the thickness Z of the specimenswherein it should be noted that the bottom BO of the specimen is theinner-wire side.

[0071] It should also be evident from FIG. 6 that the relativepercentage of fines at the top TO of the specimen (i.e. the outer-wireside) is lower than at the bottom BO of the specimen, but stillrelatively high.

[0072] In case that two fibre webs with the fines distributionillustrated in FIG. 6 are couched together with their inner-wire sidesBO against each other, a multilayered paper or paperboard web accordingto the invention with a very high internal bond strength is obtainedafter pressing and drying. Furthermore, the relatively high finescontent at the outer-wire sides TO makes it possible to reach a highinternal bond strength also after couching to further layers.

[0073] Furthermore, it should be noted that the arrangements 100; 200;300; 400; 500 in FIGS. 1-4 and 7 all are provided with so-called formingboards 132, 133; 232, 233; 332, 333; 432, 433; 532, 533 with supportfoils inside the inner-wire loops downstream their forming rolls 109,111; 209, 211; 309, 311; 409, 411; 509, 511. However, the wiretables/support foils shown in the figures do not provide any drainagefunction.

What is claimed is:
 1. An apparatus for forming a multilayered paper orpaperboard web from stock suspensions containing papermaking fibers andfines, the apparatus comprising: a first forming unit including aforming roll, an inner wire, and an outer wire, the first forming unitbeing operable to produce a fiber web having a higher concentration offines at one side than at an opposite side of the web; a second formingunit including a forming roll, an inner wire, and an outer wire, thesecond forming unit being operable to produce a fiber web having ahigher concentration of fines at one side than at an opposite side ofthe web; and a couch device for couching together the webs from thefirst and second forming units, wherein the forming units and couchdevice are arranged relative to one another so as to place the sides ofthe webs having the higher concentration of fines in contact with eachother for couching together to form the multilayered web.
 2. Theapparatus of claim 1 , wherein each of the forming units is arrangedsuch that the inner and outer wires pass along a portion of thecircumference of the forming roll to form an initial curved section of aforming zone in which initial dewatering takes place so as to form a webhaving an inner wire side against the inner wire and an outer wire sideagainst the outer wire, wherein said curved section is followed by asection having additional drainage members operable to further dewaterthe web, and wherein the forming roll is an open forming roll operableto produce a higher concentration of fines at the inner wire side of theweb than at the outer wire side, and the additional drainage members areoperable to further dewater the web while substantially maintaining thehigher concentration of fines at the inner wire side of the web.
 3. Theapparatus of claim 2 , wherein the additional drainage members arearranged adjacent the outer wires for dewatering the webs from the outerwire sides thereof.
 4. The apparatus of claim 3 , wherein the additionaldrainage members include vacuum suction boxes.
 5. The apparatus of claim4 , wherein at least one of the vacuum suction boxes, in addition todewatering the respective web, is arranged to transfer the web from theinner wire onto the outer wire.
 6. The apparatus of claim 4 , wherein atleast one of the vacuum suction boxes, in additional to dewatering therespective web, is arranged to join the webs together to form themultilayered web.
 7. The apparatus of claim 2 , further comprisingvacuum transfer devices arranged for joining the webs together to formthe multilayered web.
 8. The apparatus of claim 7 , wherein at least oneof the vacuum transfer devices also comprises an additional drainagemember for continued dewatering of the web.
 9. The apparatus of claim 2, wherein at least the forming rolls, additional drainage members, andwires are identical between the first and second forming units.
 10. Theapparatus of claim 9 , further comprising drive rolls and tensioningrolls for the forming units, wherein the drive rolls and tensioningrolls are identical between the first and second forming units.
 11. Theapparatus of claim 2 , further comprising a first vacuum transfer devicearranged for transferring the web formed in the first forming unit fromthe inner wire onto the outer wire thereof, a second vacuum transferdevice arranged for transferring the web formed in the second formingunit from the inner wire onto the outer wire thereof, and a third vacuumtransfer device arranged for transferring the web formed in the secondforming unit from the outer wire thereof onto the web formed in thefirst forming unit.
 12. The apparatus of claim 2 , further comprising abottom layer forming unit for forming a bottom layer to be joined withthe webs formed in the first and second forming units, the bottom layerforming unit having at least one bottom wire and a headbox for formingthe bottom layer on the bottom wire, and further comprising at least onevacuum transfer device arranged for transferring the web formed in thefirst forming unit onto the bottom layer on the bottom wire.
 13. Theapparatus of claim 12 , further comprising at least one additionalvacuum transfer device arranged for transferring the web formed in thefirst forming unit from the outer wire onto the inner wire thereof. 14.The apparatus of claim 2 , further comprising a top layer forming unitfor forming a bottom layer to be joined with the webs formed in thefirst and second forming units, the top layer forming unit having atleast one top wire and a headbox for forming the top layer on the topwire, and further comprising at least one vacuum transfer devicearranged for transferring the top layer onto the web formed in thesecond forming unit.
 15. A method for forming a multilayered paper orpaperboard web from stock suspensions containing papermaking fibers andfines, the method comprising: forming a first fiber web having a higherconcentration of fines at one side than at an opposite side thereof, thefirst web being formed in a first twin-wire forming unit; forming asecond fiber web having a higher concentration of fines at one side thanat an opposite side thereof, the second web being formed in a secondtwin-wire forming unit; and couching the webs together with the sideshaving a higher concentration of fines in contact with each other. 16.The method of claim 15 , wherein each forming unit comprises a formingroll, an inner wire, and an outer wire arranged such that the inner andouter wires pass along a portion of the circumference of the formingroll to form an initial curved section of a forming zone in whichinitial dewatering takes place so as to form a web having an inner wireside against the inner wire and an outer wire side against the outerwire, wherein continued dewatering of the web is performed after saidcurved section by additional drainage members operable to furtherdewater the web, and wherein the forming roll is an open forming rolloperable to produce a higher concentration of fines at the inner wireside of the web than at the outer wire side, and the additional drainagemembers are operable to further dewater the web while substantiallymaintaining the higher concentration of fines at the inner wire side ofthe web.
 17. The method of claim 16 , wherein the initial dewateringresults in the webs having a dryness of about 2 to 3 percent.
 18. Themethod of claim 17 , wherein the continued dewatering results in thewebs having a dryness greater than about 6 percent.
 19. The method ofclaim 16 , wherein the continued dewatering occurs primarily in adirection toward the outer wire sides of the webs.
 20. The method ofclaim 16 , wherein the forming rolls have a diameter of about 1.4 to 1.8m and the wires are wrapped about the forming rolls with a wrap angle ofat least about 80°.
 21. The method of claim 16 , wherein the initialdewatering results in the webs having a dryness of about 6 percent, andthe continued dewatering results in the webs having a dryness greaterthan about 6 percent.
 22. The method of claim 21 , wherein the continueddewatering is performed by at least one vacuum transfer devices.
 23. Themethod of claim 22 , further comprising using vacuum transfer devices toperform additional dewatering after the continued dewatering so as toachieve a dryness of the webs greater than about 8 percent before thewebs are couched together to form the multilayered web.
 24. The methodof claim 15 , wherein the first web formed in the first forming unit istransferred from the inner wire onto the outer wire thereof which isarranged to pass adjacent to the outer wire of the second forming unit,and wherein the second web formed in the second forming unit istransferred from the inner wire onto the outer wire thereof and istransferred from the outer wire of the second forming unit onto thefirst web that is on the outer wire of the first forming unit.
 25. Themethod of claim 15 , further comprising forming a bottom layer on atleast one bottom wire, and thereafter transferring the first web ontothe bottom layer.
 26. The method of claim 25 , wherein the first web,prior to being transferred onto the bottom layer, is transferred fromthe outer wire onto the inner wire of the first forming unit.
 27. Themethod of claim 15 , further comprising forming a top layer on at leastone top wire and thereafter transferring the top layer onto the secondweb.